As of this writing, wireless devices have become ubiquitous in American society. Pagers, cellular telephones, personal digital assistants and the like (collectively “mobile terminals”) can be found in home and business environments. Likewise, the availability of affordable wireless routers has caused a surge in the number of wireless local area networks. Common to most of these devices are wireless transmitters and receivers. In these wireless transmitters and receivers, it is common to have an on-chip voltage controlled oscillator (VCO), a tuned amplifier, a power amplifier, or an up-conversion mixer amidst the electronic components that form such devices. These elements typically rely on inductors for at least part of their functionality.
As mobile terminals become more common, the various communication industries have perceived that the public believes that smaller is better when it comes to mobile terminals. As a result, there is an industry-wide effort to miniaturize components within mobile terminals. Some components are inherently difficult to miniaturize. One such component that resists miniaturization is an inductor, which, as noted, can be present in a VCO, a tuned amplifier, a power amplifier, or an up-conversion mixer. An early effort to create smaller inductors was the use of a differential inductor.
A differential inductor is a special case of an inductor, which is physically designed for symmetry or near-symmetry between its two ends (i.e., a “symmetric” inductor), in which the two ends are used as inputs for AC signals that are out of phase. In this document, the term “differential inductor” is used as a synonym for “symmetric inductor” with regard to purely physical design properties, as well as being used in its usual sense of defining a driving condition for the inductor's inputs. Specifically excluded from the definition of “differential inductor” are two separate, non-intertwined inductors which happen to be symmetric with respect to each other (usually mirrored) and/or happen to be differentially driven. While the literature sometimes refers to such a driven pair of inductors as a differential inductor, for the purposes of this document, such an inductor is referred to herein as a differentially driven pair of inductors.
Differential inductors are typically laid out with overlapping, oppositely wound (with respect to the respective inputs) coils containing one or more loops. The oppositely wound coils effectively almost double the inductance of the element in approximately the same space that a single coil inductor would occupy. The oppositely wound coils of the differential inductor position oppositely charging loops next to each other. This positioning creates a large effective capacitance (CEFF) within the differential inductor. This effective capacitance lowers the self-resonance frequency (FSR) of the differential inductor according to 2πFSR=1/√(LCEFF), where L is the total inductance of one differential branch, in effect limiting the frequency range over which the differential inductor can operate.
To address this limited operating range, conventional circuit design spreads the loops of the inductor farther apart; however, this has at least two disadvantages. First, spreading the loops consumes more space, which, as already mentioned, is at a premium in mobile terminals. Second, when the loops are spread apart, the magnetic coupling of the loops decreases, so that the inductor has a lower inductance. This lower inductance is offset by adding windings, which also takes up more space. Thus, there is a need for an improved differential inductor that does not take up excessive amounts of space within the mobile terminal.